ckeditor - in which way is I allowed to use it?

I am a bit unsure about the license in CKeditor. When I am making a Content Management System which I am "selling" to customers, how am I allowed to use the CKeditor in this System?

The payment method is about setting up 开发者_如何转开发the system - the System it self is "free".

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As long as you provide the source code of your work either directly or upon request, there is nothing stopping you from selling a product that includes software licensed under the GPL.